The William Herschel Telescope (WHT) is a 4.20-metre (165 in) optical and near-infrared reflecting telescope located at the Roque de los Muchachos Observatory on the island of La Palma in the Canary Islands, Spain. The telescope, which is named after William Herschel, the discoverer of the planet Uranus, is part of the Isaac Newton Group of Telescopes. It is funded by research councils from the United Kingdom, the Netherlands and Spain. At the time of construction in 1987, the WHT was the third largest single optical telescope in the world. It is currently the second largest in Europe, and was the last telescope constructed by Grubb Parsons in their 150-year history. The WHT is equipped with a wide range of instruments operating over the optical and near-infrared regimes. These are used by professional astronomers to conduct a wide range of astronomical research. Astronomers using the telescope discovered the first evidence for a supermassive black hole (Sgr A*) at the centre of the Milky Way, and made the first optical observation of a gamma-ray burst. The telescope has 75% clear nights, with a median seeing of 0.7".
History The WHT was first conceived in the late 1960s, when the 3.9 m (150 in) Anglo-Australian Telescope (AAT) was being designed. The British astronomical community saw the need for telescopes of comparable power in the Northern Hemisphere. In particular, there was a need for optical follow-up of interesting sources in the radio surveys being conducted at the Jodrell Bank and Mullard observatories (both located in the UK), which could not be done from the Southern Hemisphere location of the AAT. The AAT was completed in 1974, at which point the British Science and Engineering Research Council began planning for a group of three telescopes located in the Northern Hemisphere (now known as the Isaac Newton Group of Telescopes, ING). The telescopes were to be a 1.0 m (39 in) (which became the Jacobus Kapteyn Telescope), the 2.5 m (98 in) Isaac Newton Telescope which was to be moved from its existing site at Herstmonceux Castle, and a 4m class telescope, initially planned as a 4.5 m (180 in). A new site was chosen at an altitude of 2,344 m (7,690 ft) on the island of La Palma in the Canary Islands, which is now the Roque de los Muchachos Observatory. The project was led by the Royal Greenwich Observatory (RGO), who also operated the telescopes until control passed to an independent ING when the RGO closed in 1998. By 1979 the 4 m was on the verge of being scrapped due to a ballooning budget, whilst the aperture had been reduced to 4.2 m (170 in). A panel known as the Tiger Team was convened to reduce the cost; a re-design cut the price-tag by 45%. Savings were primarily made by reducing the focal length of the telescope – which allowed the use of a smaller dome – and relocating non-essential functions outside the dome to a simpler (and thus cheaper) rectangular annexe. In the same year, the Isaac Newton Telescope was moved to Roque de los Muchachos Observatory, becoming the first of the Isaac Newton Group of Telescopes. In 1981 the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (Netherlands Organization for Scientific Research, NWO) bought a 20% stake in the project, allowing the WHT to be given the go-ahead. That year was the 200th anniversary of the discovery of Uranus by William Herschel, so it was decided to name the telescope in his honour. Construction of the telescope was by Grubb Parsons, the last telescope that company produced in its 150-year history. Work began at their factory in Newcastle upon Tyne in 1983, and the telescope was shipped to La Palma in 1985 (the two other telescopes of the Isaac Newton Group began operating in 1984). The WHT saw first light on 1 June 1987; it was the third largest optical telescope in the world at the time. The total cost of the telescope, including the dome and the full initial suite of instruments, was £15M (in 1984, equivalent to £49M in 2025); within budget once inflation is taken into account.
Design
Optics The telescope consists of a 4.20 m (165 in) f/2.5 primary mirror made by Owens-Illinois from Cervit, a zero-expansion glass-ceramic material, and ground by Grubb Parsons. The mirror blank was produced in 1969 as one of a set of four, along with those for the AAT, CFHT and Blanco telescopes, and was purchased for the WHT in 1979, ten years after it was made. The primary is solid and un-thinned, so no active optics system is required, despite its weight of 16.5 tonnes (16.2 long tons). The mirror support cell holds the main mirror on a set of 60 pneumatic cylinders. Even under the most extreme loading (with the telescope pointing at the horizon, so the mirror is vertical) the shape of the mirror changes by only 50 nanometres (2.0×10−6 in); during normal operation the deformation is much smaller. In its most usual configuration, a 1.00 m (39 in) hyperbolic secondary mirror made of Zerodur is used to form a Ritchey Chretien f/11 Cassegrain system with a 15 arcmin field of view. An additional flat fold mirror allows the use of any one of two Nasmyth platforms or two folded Cassegrain stations, each with 5 arcmin fields of view. The telescope sometimes operates in a wide-field prime focus configuration, in which case the secondary is removed and a three element field-correcting lens inserted, which provides an effective f/2.8 focus with a 60 arcmin field of view (40 arcmin unvignetted). Changing between the Cassegrain and Nasmyth foci takes a matter of seconds and may be done during the night; switching to and from prime focus requires replacing the secondary mirror with a prime focus assembly during daytime (the two are mounted back-to-back) which takes around 30 minutes. A Coudé focus was planned as a later addition, to feed an optical interferometer with another telescope, but this was never built. A chopping f/35 secondary mirror was planned for infrared observations, but was placed on hold by the cost-saving re-design and never implemented.
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